This invention generally relates to electronic registration and, more particularly, to a system to improve image registration by electronic compensation of raster output scanner beam scan trajectory distortions.
Electrophotographic marking is a well-known and commonly used method of copying or printing documents. In general, electrophotographic marking employs a charge-retentive, photosensitive surface, known as a photoreceptor, that is initially charged uniformly. In an exposure step, a light image representation of a desired output focused on the photoreceptor discharges specific areas of the surface to create a latent image. In a development step, toner particles are applied to the latent image, forming a toner or developed image. This developed image on the photoreceptor is then transferred to a print sheet on which the desired print or copy is fixed.
The electrophotographic marking process outlined above can be used to produce color as well as black and white (monochrome) images. Generally, color images are produced by repeating the electrophotographic marking process to print two or more different image layers or color image separations in superimposed registration on a single print sheet. This process may be accomplished by using a single exposure device, e.g. a raster output scanner (ROS), where each subsequent image layer is formed on a subsequent pass of the photoreceptor (multiple pass) or by employing multiple exposure devices, each writing a different image layer, during a single revolution of the photoreceptor (single pass). While multiple pass systems require less hardware and are generally easier to implement than single pass systems, single pass systems provide much greater print speeds.
In generating color images, the ability to achieve precise registration of the image layers is necessary to obtain printed image structures that are free of undesirable color fringes and other registration errors. Precise registration of image layers in a single pass machine requires precise registration from one ROS to the next. One major cause of misregistration in multiple ROS systems is the differences in the beam scan trajectory of each ROS in the imaging system.
In general, a conventional ROS repeatedly scans a data modulated light beam over a photoreceptor surface in accordance with a predetermined raster scanning pattern to generate an image. Typically, a conventional ROS includes a laser diode or similar device to generate a light beam that is modulated in response to received data. The ROS further includes a rotating polygonal mirror block to repeatedly scan the light beam across the photoreceptor. As the photoreceptor is advanced in a process direction, the ROS repeatedly scans the modulated light beam across the surface of the photoreceptor in a fastscan direction that is orthogonal to the process direction.
Ideally, each scan of the light beam across the photoreceptor (generally identified herein as a beam scan) traces a straight line across the surface of the photoreceptor that is substantially normal to the movement of the photoreceptor in the process direction. However, variations in the angular speed of the rotating polygonal mirror as well as variations in the geometry of the sidewalls or facets of the rotating mirror can introduce pixel positioning errors which distort the trajectory of each beam scan. Typically, each ROS introduces different pixel positioning errors that distort its beam scan. Thus, in a machine with more than one ROS, each ROS will likely have a different beam scan trajectory.
To achieve the registration necessary to generate color images that are free of undesirable registration errors, the beam scan trajectory of each ROS must be within a relatively tight bound such as xc2x15 microns of the beam scan trajectory of every other ROS. Such tight registration tolerances are very difficult and very expensive to achieve solely by opto-mechanical means within the ROS. Systems for compensation and/or correction of beam scan distortions to improve registration errors have been proposed. However, many of these proposed systems correct only one type of distortion and often are themselves complex and expensive to implement.
The following references may be found relevant to the present disclosure.
U.S. Pat. No. 5,430,472 to Curry discloses a method and apparatus for eliminating misregistration and bowing by controlling a composite light intensity profile and phase shifting of a spatial location at which the light intensity profile crosses a xerographic threshold in a two dimensional high addressability printer operating in an overscan mode.
U.S. Pat. No. 5,732,162 to Curry discloses a system for correcting registration errors in a printer with subscan precision. The system includes a memory device for storing sequential rasters of image data and an interpolator coupled to the memory device. The interpolator uses the rasters of image data from the memory device in conjunction with multiplication factors to calculate an interpolated resample value.
In accordance with the present invention, there is provided a system for generating a warped contone image that compensates for beam scan trajectory distortions. The system includes an image buffer for storing a portion of a received contone image. An interpolation coefficient generator provides interpolation coefficients associated with a warped pixel within the warped contone image in response to an input identifying the warped pixel. A pixel interpolator is coupled to the image buffer and the interpolation coefficient generator and provides an output signal that identifies the warped pixel within the warped contone image. The pixel interpolator further retrieves pixels from the contone image stored in the image buffer using the interpolation coefficients and generates the warped pixel by combining the retrieved pixels into a single pixel value.
In accordance with another aspect of the present invention, there is provided a printing system including a print engine that generates an output document in response to print ready data and an image processing system operating on image data representing an image to be printed to generate the print ready data supplied to the print engine. The image processing system includes a warping processor for generating a warped contone image that compensates for pixel positioning errors introduced by the print engine. The warping processor includes an image buffer coupled to receive and store a portion of the contone image, an interpolation coefficient generator providing an interpolation coefficient associated with a warped pixel within the warped contone image in response to an input signal identifying the warped pixel and a pixel interpolator coupled to the image buffer and the interpolation coefficient generator. The pixel interpolator provides an output signal identifying the warped pixel, retrieves pixels from the contone image stored in the image buffer in response to the interpolation coefficient associated with the warped pixel and generates the warped pixel by combining the retrieved pixels into a single pixel value.
In accordance with yet another aspect of the present invention, there is provided a method of realigning pixels within continuous tone image data to compensate for distortion in a beam scan trajectory of a first output scanner and improve image layer registration. The method comprises the steps of: receiving said continuous tone image data, said received continuous tone image data comprising a plurality of scanlines; identifying a warped pixel within a warped scanline; identifying pixels within said received continuous tone image data that compensate for said distortion in said beam scan trajectory; retrieving said identified pixel from said received continuous tone image data based on said desired output position; and generating said warped pixel from said retrieved pixels.